The development of practical magnetoelectric (ME) multiferroic devices for industry is concentrated on two embodiments: The first is for weak-field sensors for applications such as mine detection in harbors or related military devices. In this case practical devices favor sandwich-type bilayer or multilayer composites of ferroelectrics such as lead zirconate-titanate (PZT) with magnets such as terphenyl-d but this configuration yields low ME voltage coefficients which is not suitable for weak field sensor applications. The second is for random access memory elements where the ability to switch magnetization with applied electric fields (and vice versa) would permit the combination of fast (possibly sub-nanosecond) WRITE operation with nondestructive magnetic READ operation. Such a combination would produce formidable competition for FLASH EEPROMs, particularly in view of the fact that magnetoelectric RAMs would operate at <1.0V, an international target for all microelectronics in the next decade which FLASH devices are unlikely to meet without cumbersome internal charge pumps.
However, most multiferroics to date (such as terbium manganites TbMnO3 or TbMn2O5) switch only nC/cm2 with applied magnetic fields—approximately 1000× too small for reliable discrimination between “1” and “0” states. Moreover, they operate only at cryogenic temperatures. BiFeO3 might operate at room temperature, but it exhibits a weak magnetism (ca. 0.03 emu/cc), high leakage current, and it does not permit switching of neither magnetization with electric fields nor polarizations with magnetic fields. There are several other magnetically frustrated systems that have been identified which show gigantic magnetoelectric effects at low temperature. Among them, MnWO4 shows external magnetic field dependence of its ferroelectric loops below room temperature. Similarly, high-temperature multiferroic behavior has been reported in PbFe0.5Ti0.5O3 and in PbFe2/3W1/3O3/PbTiO3. Others have studied the relaxor properties of PFW and PFW-PT ceramics.
Clearly a new line of thinking is required to overcome the intrinsic limitations of direct linear magnetoelectric (ME) coupling of form PM in the free energy, where P is polarization and M, magnetization. There have been recent efforts by researchers throughout the world to produce electric switching of magnetizations or magnetic switching of polarizations in multiferroic magnetoelectrics. Generally, schemes for switching from +M to −M with electric field (E) or conversely from +Pr to −Pr with magnetic field (H) have been examined. In the present state of the art there is a need for a new single-phase material that can exhibit magnetoelectricity (not necessarily linear) at room temperature.